The invention relates to FM/CW or xe2x80x9cfrequency modulation/continuous wavexe2x80x9d type radio altimeters. These radio altimeters periodically send out a continuous wave, linearly modulated in frequency between two boundary values, in other words, a saw-toothed wave. When this wave is received by the radio altimeter after being reflected by the ground, it has a delay 2h/c with respect to the wave sent, where h is the height of the radio altimeter from the ground and c is the speed of light. Since the modulation is linear, this delay is proportional to the beat frequency, f, obtained by the mixing of the transmission signal and the reception signal. If we know the difference dF between the two boundary values and the duration Td taken by the modulation to pass from one boundary value to the other, it is then possible to compute the height h by the formula:
h=Td*f*c/(2*dF)
In reality, the mixture of the waves sent and received does not give the beat frequency alone but an entire spectrum of frequencies. The different methods used to process these frequencies makes it possible to extract the frequency representing the height to be measured. A method of this kind is. described, for example, in the French patent No. 2 750 766.
However, in these prior art radio altimeters, there is no means, apart. of the use of a test stand, of performing a calibration to ascertain that the measurements made by the radio altimeter are precise. Now, it would be valuable to be able to make this verification, and to do so even during the working of the radio altimeter.
The present invention is aimed at proposing a method which, for the cost of a relatively minor adaptation of the electronic circuitry of the radio altimeter, enables precise calibration at all times.
This is obtained by the production, in the radio altimeter, in addition to the transmission signal, of a pseudo reception signal and by the checking of the value of the pseudo-height measured by the radio altimeter with reference to this pseudo-signal.
For this purpose, the invention proposes a method of calibration designed for an FM/CW type radio altimeter comprising a linear oscillator to give a saw-toothed signal, with a link to a transmitting antenna, a mixer circuit with two links respectively towards the oscillator and towards a receiving antenna, and a processing device to process the output signals from the mixer circuit, wherein the method consists in:
injecting an auxiliary saw-toothed signal with a given saw-tooth slope, Td/dF, into the link between the receiving antenna and the mixer, the sawteeth of this signal being synchronous with those of the linear oscillator, and the plateau of the sawteeth of this signal being offset with respect to the plateau of the sawteeth of the linear oscillator by a given value f,
obtaining, by modification or non-modification of the duration of modulation of the sawteeth of the linear oscillator or the dF of the linear oscillator, of a beat at the frequency f between the sawteeth of the oscillator and of the auxiliary signal so that the processing device gives a measured height hxe2x80x2, and
comparing this measured height with the standard height obtained by computation according to the formula h=f*c*Td/(dF*2), where c is the speed of light.
The invention also proposes an FM/CW type radio altimeter comprising, firstly, in series, a control device, a linear oscillator and first coupling elements to couple the oscillator to a transmitting antenna and, secondly, a mixer circuit with a first input coupled to the linear oscillator and a second input, second coupling elements to couple a receiving antenna to the second input of the mixer circuit and a processing device to process the output signals from the mixer circuit wherein, to perform a calibration of the radio altimeter, it comprises an auxiliary linear oscillator and a directional coupler connected in series, the auxiliary oscillator being controlled by the control device and the coupler being inserted into the second coupling elements.